Microwave gaseous switch tube for coaxial transmission lines



E. J. COOK June 27, 1967 MICROWAVE GASEOUS SWITCH TUBE FOR COAXIAL TRANSMISSION LINES Filed May 13, 1965 RE 4 WT I I- (3 LS mm v fl INVENTOR. EDWARD J. COOK FIGS ATTORNEY United States Patent O 3 328,729 MICROWAVE GASIEOUS swrrcn TUBE FOR COAXIAL TRANSMISSION LINES Edward J. Cook, South Hamilton, Mass., assiguor to Varian Associates, Palo Alto, Calif., a corporation of California Filed May 13, 1965, Ser. No. 455,507 7 Claims. (Cl. 33313) The present invention relates in general to gaseous switch tubes and more particularly to an improved high power microwave ATR or Pre-TR coaxial line gaseous switch tube assembly employing folded cylinder type gaseous switch elements. Such improved coaxial line TR tubes are especially useful at UHF frequencies in pulsed radars.

Heretofore the folded cylinder has been extensively used in hollow rectangular waveguide transmission lines as a gaseous switch element. Such folded cylinder elements have been found to be especially desirable for ATR or Pre-TR switch elements because of their high power handling capability, i.e., tens of megawatts, and fast recovery time, i.e., tens of microseconds. Rectangular waveguide TR devices are generally unsatisfactory at low microwave frequencies, i.e., UHF 2004000 mc., because of the very large physical size that the waveguide components take at these frequencies. For example, a 200 mc. rectangular waveguide would have a broad wall width of approximately two feet.

On the other hand, a coaxial transmission line for megawatts of power at 200 me. would have an inner diameter of approximately five inches.

Prior attempts to use folded cylinder tubes with the dominant- TEM mode in high power UHF frequency coaxial lines have not been too satisfactory. More specifically these prior coaxial folded cylinder switch tubes were not satisfactory because the folded cylinders had been fitted with conductive electrodes bonded to the folded cylinder envelope such electrodes being arranged to make physical and electrical contact with spring fingers carried from the inner and outer conductors-of the coaxial line. In operation,-R.F. arcing occurred across the spring fingers causing welding thereof to the electrodes of the folded cylinder thereby preventing later replacement-of the folded cylinders. In addition, R.F. arcing occurred across the envelope of the folded cylinder where the electrodes were bonded thereto causing localized overheating and puncture of the folded cylinder in this vicinity thereby rendering same inoperative.

The principalobject of thepresent invention is to-provide an improved high power folded cylinder UHF coaxial Pre-TR device which is characterized by high power handling capability and long operating life.

One feature of the present invention is the provision of a pair of elongated coaxially aligned and axially spaced apart sleeve conductor member portions, one sleeve portion carried from the outer conductor and the other sleeve portion carried from the inner conductor. Said sleeve portions being adapted to receive afolded cylinder gaseous switch tube internally thereof and extending between said elongated spaced sleeve portions for shorting said coaxial line upon said switch tube becoming conducting in use whereby said elongated sleeves serve to reduce the shorting electric displacement field intensity to below the arcing level when transferring the shorting current between the inner and outer conductors whereby arcs between the sleeves and the folded cylinder are prevented and thus the operating life extended in use.

Another feature of the present invention is the same as the preceding feature wherein a plurality of said pairs of ice.

coaxial sleeve portions and folded cylinders are used to carry the shorting displacement current.

Other features and advantages will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a longitudinal sectional view of a coaxial line TR;

FIG. 2 is a transverse sectional view of the structure of FIG. 1 taken along line 22;

FIG. 3 is a fragmentary sectional view in reduced scale of the structure of FIG. 1 taken along line 33 in the direction of the arrows;

FIG. 4 is an enlarged fragmentary view of a portion of the structure of FIG. 2 delineated by line 4-4.

Referring now to the figures there is shown a coaxial line TR tube incorporating the features of the present invention. More particularly, a coaxial transmission line 1 comprising an inner conductor 2 surrounded by an outer conductor 3 includes an enlarged diameter gaseous switch tube assembly portion 4.

The gas switch tube assembly 4 includes a pair of transversely displaced elongated cylindrical conductive sleeve members 5, as of copper, passing through a pair of notched-out portions of the inner conductor 2. The sleeves 5 are parallel to each other and extend at one end closer to the outer conductor 3 than at their other end. A corresponding pair of transversely disposed metallic conductive sleeves 6 pass through the outer conductor 3. The sleeves 6 are axially spaced apart from the corresponding sleeves 5 and are disposed in coaxial alignment therewith to form a discharge initiation gap 7 therebetween.

A pair of folded cylinder type gas filled switch tubes 8 are disposed within the sleeves 5 and 6. The folded cylinders are dimensioned in diameter such that there is a cold clearance of approximately 0.003 to 0.005", on the diameter between the tubes 8 and the inside wall of the sleeves 5- thus defining a cylindrical air filled gap 10 between the folded cylinder 8 and the sleeve 5. The folded cylinders 8 are of the conventional design and include a gas reservoir 9 at one end and are spaced apart from the outer conductor 3 of the coaxial line at their other ends as by for example /2" via a dielectric plug 11 as of Teflon plastic. The folded cylinder is dimensioned of such a length that the reservoir is positioned out of the sleeve 6.

The plug 11 closes off an elongated access port 12 in the outer conductor useful for inserting and removing the folded cylinders 8. The plug 11 includes a rubber O ring 13 which seals the plug 11 to the inside wall of the port 12. The plug 11 includes a metallic cover flange 14 held to the outer conductor 3 via screws 15.

, A pair of tubular metallic caps 16 are threaded over the outer ends of outer conductor sleeve member 3 for retainingthe folded cylinders 8 within their respective sleeves 5 and 6. Each cap 16 includes a compressible spring 17 and a dielectric cushion member 18 as of silicone rubber to absorb mechanical shock and vibration.

The conductive .elements, i.e., sleeves 5 and 6, within the switch tube assembly are dimensioned and spaced to preserve the same characteristic impedance Z as the remaining portions of the coaxial line in which the switch tube assembly is inserted.

More specifically this accounts for the bulge in the outer conductor 3 in the region surrounding the sleeves 5 and 6. The bulge is necessary to'decrease some of the distributed capacity between the sleeves 5 and the outer conductor 3 to compensate for the increased distributed capacity between the opposed ends of the sleeves 5 and 6 at the discharge gap 7. The bulge permits retention of a constant distributed capacity per unit length of the co- 7 axial line, in spite of the switch tube structure therein thereby maintaining a nearly constant characteristic impedance for the line 1 and preventing unwanted reflections of energy when the gas switch tube elements are non-conductive. The coaxial line 1 is pressurized with air to a pressure of 20 p.s.i.

In operation, wave energy traveling down the coaxial line in the TEM mode upon reaching the gas switch tube assembly 4 has its electric field concentrated in the dis charge gap 7. Below the threshold firing power of the gas tubes 8 the wave energy passes through the switch tube assembly 4 substantially unattenuated and without substantial reflection.

Upon reaching the switch tube firing power level the wave energy, traveling down the coaxial line 1, will cause the gas fill within the folded cylinders 8 to break down and become conductive. When this happens the gas switch tubes 8 look electrically like a pair of large hollow conductive members connected between the outer conductor 3 and the inner conductor 2 via a pair of cylindrical coaxial capacitors. The coaxial capacitors are formed by the end portions of the folded cylinders 8 which are axially coextensive With the inner conductive surfaces of the sleeves 5 and 6. The gap of the coaxial capacitors is formed by the 0.003" to 0.005 cylindrical air gap 10 plus the outer cylindrical dielectric Wall of the folded cylinders 8 :between the inner diameter of the sleeves 5 and 6 .and the outer diameter of the hollow cylindrical gas fill region of the folded cylinders 8. These capacitor portions, i.e., the internal sleeve area, then must be dimensioned to have suflicient area such that the displacement current electric field intensity E in the capacitors does not exceed the breakdown field intensity or dielectric strength for the gas fill of the capacitor gap region. In the case of air at 20 p.s.i. this breakdown field intensity is on the order of 30 kv./cm. Thus the area of the capacitors must increase with increasing operating power level to prevent arcing or breakdown of the air gap 10. If this air gap 10 breaks down localized overheating of the folded cylinder 8 is obtained causing fracture or puncture thereof thereby rendering same inoperative. Higher power levels may be easily accommodated by arranging larger clusters of the folded assemblies across the coaxial line.

Briefly, the needed area A and thus the internal dimensions for the sleeves 5 and 6 can be computed as follows:

The line current 1;, may be found from,

where P is the power to be reflected and Z is the characteristic line impedance of the coaxial line.

The short circuit current I carried by the gas tubes, and indicated in FIG. 4 by current density vectors J is 21;, and,

where A is the total internal area of the sleeve at one end on the gas tube, 11 is the number of gas tubes, and D is the time rate of change in electric flux density D indicated in FIG. 4 by the vectors D. D is defined by,

Thus substituting values of D obtained from Eq. 3 into Eq. 2 the area for each of the sleeves can be computed knowing the power to be reflected and the characteristic impedance Z of the line 1.

The above equations hold for frequencies wherein the lengths of the sleeves 5 and 6 are only a small fraction of a wavelength, i.e., less than k within the coaxial line.

A gaseous switch tube assembly built according to the above teachings utilizing two folded cylinder switch tubes 8 effectively shorted hundreds of kilowatts of peak power at an average power of tens of kilowatts at UHF frequencies between 200 and 400 me. A similar switch tube assembly using a cluster of several inner conductor sleeves 5 may be built to short circuit megawatts of peak power and hundreds of kilowatts of average power in the UHF band.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A coaxial line switch tube apparatus, including means forming a section of coaxial line having an inner conductor portion and an outer conductor portion surrounding said inner conductor portion, said coaxial line having an axis in the direction of power transfer along said line, said inner conductor including a sleeve portion having an axis transversely directed of the axis of the coaxial line, said sleeve portion being fixedly secured to said inner conductor portion and forming an integral part thereof, said outer conductor portion having a corresponding sleeve portion with an axis coaxially aligned with the axis of said inner conductor sleeve portion, said sleeve portion being fixedly secured to said outer conductor portion and forming an integral part thereof, said inner and outer conductor sleeve portions being spaced apart along their common axis to define an R.F. discharge gap therebetween, said sleeve portions being adapted to accommodate a microwave switch tube therewithin and to capacitively couple current to said switch tube to short said coaxial line when power in said line exceeds a predetermined level.

2. The apparatus according to claim 1 including a folded cylinder gas switch tube disposed within and subtending the gap between said inner and outer coaxially aligned sleeve portions.

3. The apparatus according to claim 1 including means forming a dielectric plug disposed in said outer conductor of said coaxial line in axial alignment with the axis of said coaxially aligned inner and outer sleeve portions and on the remote side of said inner conductor sleeve from said outer conductor sleeve for capturing one end of said switch tube.

4. The apparatus according to claim 1 wherein said outer conductor portion includes a plurality of said outer conductor sleeve portions, said inner conductor includes a plurality of said inner conductor sleeve portions 00- axially aligned with said plurality of outer conductor sleeve portions to define a plurality of RF. discharge gaps therebetween and to accommodate a plurality of microwave switch tube means within said sleeve portions.

5. The apparatus according to claim 1 wherein each of said inner conductor sleeve portions which is axially coextensive with the switch tube has an area greater than A defined by the following expression:

P a 0:630 KV./cm.

where P is the maximum rated peak power to be shorted and thus reflected by each switch tube element of the apparatus, Z is the characteristic impedance of the coaxial line, 6 is the permittivity of the region of space between 5 said inner conductor sleeve and said switch tube element, and w is the frequency in radians per second of the RF. wave energy being shorted,

6. The apparatus according to claim 1 wherein said coaxially aligned sleeve portions are elongated each having a length in excess of their diameter.

7. The apparatus according to claim 6 wherein said outer conductor portion is of enlarged internal transverse dimensions in its region adjacent said inner conductor sleeve portion to reduce the distributed capacity of certain portions of said outer conductor to compensate for the added distributed capacity between said coaxially aligned inner and outer sleeve portions whereby the reflection of wave energy by said switch tube assembly is minimized when said switch tube means is nonconductive.

References Cited UNITED STATES PATENTS 3,209,285 9/1965 Manwarren et al. 333--l3 HERMAN KARL SAALBACH, Primary Examiner.

P. L. GENSLER, Assistant Examiner. 

1. A COAXIAL LINE SWITCH TUBE APPARATUS, INCLUDING MEANS FORMING A SECTION OF COAXIAL LINE HAVING AN INNER CONDUCTOR PORTION AND AN OUTER CONDUCTOR PORTION SURROUNDING SAID INNER CONDUCTOR PORTION, SAID COAXIAL LINE HAVING AN AXIS IN THE DIRECTION OF POWER TRANSFER ALONG SAID LINE, SAID INNER CONDUCTOR INCLUDING A SLEEVE PORTION HAVING AN AXIS TRANSVERSELY DIRECTED OF THE AXIS OF THE COAXIAL LINE, SAID SLEEVE PORTION BEING FIXEDLY SECURED TO SAID INNER CONDUCTOR PORTION AND FORMING AN INTEGRAL PART THEREOF, SAID OUTER CONDUCTOR PORTION HAVING A CORRESPONDING SLEEVE PORTION WITH AN AXIS COAXIALLY ALIGNED WITH THE AXIS OF SAID INNER CONDUCTOR SLEEVE PORTION, SAID SLEEVE PORTION BEING FIXEDLY SECURED TO SAID OUTER CON- 